A rhabdomyosarcoma in an inbred strain of rat is being used to study the response of a solid transplantable tumor after irradiation by heavy ion beams produced at the Bevalac at Lawrence Berkeley Laboratory. Tumor volume response, survival of tumor cells irradiated in situ and assayed in vitro, and in vitro cell survival at several positions in the "spread-out" depth dose (Bragg) curve are being measured. The oxygen effect will also be studied in the "spread-out" region of the depth-dose curve. In addition, the kinetics of tumor cells will be measured after irradiation. These results will be compared with similar studies using 220 kV X-rays and the helium ion beam at the 184 inch cyclotron. In addition, the overall response of the tumor is being studied by the development of a computer model which includes the more important mechanisms affecting the growth and regression of the tumor.